Wireless communication systems integrated into a gateway or a decoder are increasingly multi-mode and multi-standard. They can function on at least two different frequency bands. This therefore allows them to more effectively use the spectrum of available frequencies and satisfy growing needs for capacity and robustness. To meet these needs, in wireless systems based on the IEEE-802.11a/b/g standard, the usual solution is to use two radio bands at the same time, the first operating on the 2.4 GHz band to transmit only data and the second operating on the 5 GHz band to transmit only video.
To allow both transmission channels to coexist in a single device, both frequency bands must be isolated by about 40 dB in the RF front-end circuitry, as is normally requested. The most common solutions for providing the required isolation are shown by solutions A, B, and C in FIG. 1.
Schematically, solution A consists of using two antennas, marked Antenna #1 and Antenna #2, radiating in a relatively narrow band. These antennas can be physically separated on the device's circuit board to allow maximum isolation. Each antenna is connected to a specific processing circuit on ports F1 and F2 and through a filter 1 or 2. Filters 1 and 2, which are respectively a low pass filter for filter 1 and a high pass filter for filter 2, in the shown embodiment, improve the isolation between the two antennas. The main disadvantage of this solution is its size, which is not acceptable for dual-band MIMO systems that require multiple antennas.
Solution B shows a broadband antenna, marked A3, which is connected by a single transmission line to a diplexer 3 used for separating the two bands and transmit them to the processing circuit by means of input/output ports F1 and F2. This solution is less cumbersome than solution A. However, the design of the broadband antenna is more difficult, and the 40 dB isolation is obtained by filters, which are more complex to achieve than the filters in solution A.
In solution C, a broadband antenna A4 is also used, but in this case, the antenna is connected by two access lines to the diplexer. At the two access terminals F1, F2, an isolation of about 15 to 20 dB can be obtained, which reduces the constraints on filtering.
In a known manner, a broadband antenna can be realized by using a slot antenna, such as a TSA antenna (tapered slot antenna) or Vivaldi antenna. As shown in FIGS. 2 and 3, a Vivaldi antenna is realized on a substrate S equipped with a ground plane formed by a metal layer in which is etched a slot 10 whose ends open out toward the edges of the substrate. This slot 10 can be fed by a microstrip line 11 on the substrate at a predetermined distance from the ground plane. This microstrip line 11 extends from an input/output port P1 and crosses the slot 10 substantially perpendicular to said slot, as shown in FIG. 2. In this case, the Vivaldi antenna is fed by electromagnetic coupling along a microstrip/slotline transition circuit, known as a Knorr transition. To achieve the solution C described above, one can feed a Vivaldi type slot antenna 10 with two microstrip lines 12, 13 crossing the slotline extending the Vivaldi antenna into two different coupling zones, as shown in FIG. 3. A slot antenna, such as what is shown in FIG. 3, was described in the patent application published under no. WO 06/108567. To obtain proper isolation between the two input/output ports P′1 and P′2 and normalized impedance at the coupling zones to allow the Vivaldi antenna to operate on two bands, specific dimensions between the microstrip lines 12, 13 and the coupling zone must be respected, as described in the application mentioned above.
The solutions described above have a certain number of disadvantages. Solution B, which involves cascading a conventional microstrip-to-slotline transition, as described with reference to FIG. 2, with a diplexer, causes an increase in insertion losses, namely insertion losses due to the Knorr transition and insertion losses due to junctions shared with the diplexer.
With regard to solution C, it is complicated to implement because it is based on the use of multiple quarter-wave and half-wave lines, which leads to limitations in frequency bandwidth and distance between bands.